1. Field of the Invention
The present invention relates generally to temperature switches useful for monitoring fluid process conditions in commercial and industrial fluid transport systems. In a specific, non-limiting embodiment, the invention relates to a high-diagnostic dual sensor fluid temperature switch having a one-out-of-two voting architecture.
2. Background of the Invention
In many commercial and industrial fluid transport systems, temperature switches are employed to measure the operational temperature of a process fluid disposed within the transport system. In particular, temperature switches are commonly used in sensitive fluid transport environments to detect hazardous process conditions, and to initiate the shut down of one or more process functions when a hazardous temperature condition is detected.
Generally, such temperature switches are disposed in an enclosed body that also houses an inert signal transfer medium, such as silicon, thereby effectively isolating the process fluid being measured from the temperature sensors. The housing therefore provides protection for the sensors from physical damage that could be directly caused by the heat of the process fluid. In this configuration, temperature is transmitted to the temperature sensor via the transfer medium, usually through diaphragms located on the housing body, which effectively separate the process fluid from the transfer medium while still permitting fluid temperatures to be transmitted to the sensors.
Such temperature switches typically require verification of their operational stability at regular intervals to ensure accurate calibration and system functionality, especially in sensitive fluid transport applications. The most common calibration problems affecting presently known temperature switches are “span-shift” and “zero-shift”.
Span-shift occurs when the sensitivity of a temperature sensor is no longer accurate over an entire range of predetermined operational temperatures. In contrast, zero-shift occurs when all of the temperature sensor readings are off by some constant error value within the required range. In some temperature sensors, zero-shift is much more likely to occur than span-shift; in other applications, span shift is more prevalent. Unfortunately, presently known temperature switches cannot generally detect either span-shift or zero-shift while a fluid transport system remains on-line. Instead, the switch must be removed from process service to validate calibration, thereby requiring the transport system to be taken off-line, at least with respect to the particular process function being tested, and additional operation and maintenance personnel with appropriate testing equipment.
Thus, a need exists to provide a dual sensor temperature switch having a high-diagnostic, one-out-of-two voting architecture (also referred to as “1oo2D”) capable of detecting both span-shift and zero-shift in the sensors without first removing the temperature switch from process service, and without requiring additional personnel and testing equipment. A further need exists to provide a plurality of bypass switch circuits for the dual sensor temperature switch that will enable maintenance personnel to isolate and calibrate components of the device without removing the temperature switch from process service. A still further need exists to provide a dual sensor temperature switch wherein when one of the sensors is operationally bypassed from the transport system, the process can continue to be monitored solely by the remaining on-line sensor, and wherein the ability to open the device's switch output circuit is maintained even if one of the sensors fails entirely or is otherwise rendered ineffective.